1. Field of the Invention
The present invention relates in general to the production of integrated circuit packages and more specifically to a method for molding integrated circuit semiconductor devices using lead frames.
2. Description of the Prior Art
For many years, integrated circuit (IC) devices were formed with a semiconductor IC chip that is bonded to a lead frame and then encapsulated with thermoset plastic material. These types of IC devices performed effectively but were relatively expensive to produce.
In recent times, a variety of various alternatives to the thermoset plastic encapsulation devices have been developed as disclosed in U.S. Pat. Nos. 5,214,846; 5,637,273; 5,049,526; and 5,682,673. These patents describe various methods and means for producing a relatively inexpensive IC device in which the IC chip is encapsulated by a synthetic resin rather than thermoset plastic material. The encapsulation is used to protect the semiconductor chip from external impact to insure the internal circuitry thereof.
The resin encapsulation is performed by an injection molding process that can be carried out in a highly efficient and rapid molding process. However, because of the configuration of lead frames there are certain drawbacks to the use of injection molding in this fashion. The principal problem is that it is difficult to provide a molding cavity that is sufficiently sealed so as to prevent resin leakage or flash about the leads which causes a degradation in the appearance of the finished product as well as affecting its operability.
U.S. Pat. No. 5,637,273, discloses a means and method for molding integrated circuit packages through the use of a top or bottom mold die that includes projections that are sized so that their height is equal to the thickness of the lead frame leads. During the molding operation, the mold die projections are placed between the leads of the lead frame to assist in preventing leakage of resin during the molding operation. The patent further discloses the use of a tape that is placed on the leads of the lead frame to reduce the resin leaking pressure, which pressure causes the resin to leak out of the mold. Although the invention disclosed in the ""273 Patent appears to be an improvement over prior art devices in reducing resin leakage from a mold, due to alignment problems that would most likely occur between the lead frame and the frame projections of the mold die, it seems highly likely that high speed molding operation of IC devices would not be practicable with such invention.
The present invention provides an improved method and means for allowing the high speed molding of IC devices formed by a resin injected molding process.
The present invention provides a method for using a particular injection mold for manufacturing a semiconductor device with a lead frame having a plurality of side leads. The method employs a mold formed of a top mold die and a bottom mold die, both dies including a central depression and peripheral side edges having spaced apart teeth aligned transversely to the depressions to form a series of alternating channels and projections. The teeth on one of the dies are formed with sidewalls having an inner lead enclosure portion that is sized to be equal to the thickness of the lead frame leads and an outer portion that is inclined toward the other sidewall, which results in the spacing between two adjacent teeth being wider at the outer ends of the teeth than at their inner ends. The top and bottom dies are aligned so that the projections of one fit into the channels of the other, and as they are brought into an abutting relationship the lead frame leads are guided into the lead enclosure portions to form a sealed mold cavity that extends around each of the leads to allow tighter mold die fit and reduce part flash.
A particular object and advantage of the present invention is that the inclined sidewalls of the teeth of the lower molds provide a compensating alignment mechanism for directing the lead frame leads into the channels between the lower portions of the teeth in a manner that significantly reduces pinching damage to the leads during the molding process as well as insert wear.
The foregoing and other advantages of the present invention will appear from the following description. In the description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by illustration, and not of limitation, a specific form in which the invention may be embodied. Such embodiment does not represent the full scope of the invention, but rather the invention may be employed in a variety of embodiments, and reference is made to the claims herein for interpreting the breadth of the invention.